Require at least one of several taxonomy terms or fields

Issue Summary

Would be super useful to validate node forms to force entry in at least one (or more) of multiple available taxonomies or fields.

My use case is three taxonomies where the node has to be tagged with at least one of them.
(I could accomplish this with one taxonomy with hierarchy, but trying to avoid that.)

This would be useful for CCK fields though, helping to require that some node details be provided, but not all are required.